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November 8, 2026

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Science & Health

Interstellar comet 3I/ATLAS carries methane far richer in heavy hydrogen than anything in our Solar System

Faizan Altaf

A visitor from another star has handed astronomers one of the strangest chemical measurements ever made beyond our Solar System. The interstellar comet 3I/ATLAS, which swept through our neighbourhood in 2025, carries methane loaded with far more heavy hydrogen than any comet, planet or moon we have ever measured at home.

The result comes from the James Webb Space Telescope, which watched the comet in late December 2025 as it moved away from the Sun. A team led by Nathan Roth of NASA’s Goddard Space Flight Center and American University reports in Nature Astronomy that the deuterium-to-hydrogen ratio in the comet’s methane is 3.18 percent, plus or minus 0.30 percent. That is about 13 times the value found in the only other comet where this measurement has ever been made securely, and higher than any methane measurement anywhere in the Solar System.

The finding does not just set a record. It is a clue to where the comet was born: in a cloud of gas and dust far colder, older and poorer in heavy elements than the cloud that formed our Sun and planets. It also comes with an honest warning. The number is so extreme that no published model of a planet-forming disc can explain it, and some of the comet’s other clues point in a different direction.

At a glance

What happened: The James Webb Space Telescope measured the deuterium-to-hydrogen ratio in the methane of interstellar comet 3I/ATLAS at 3.18 percent, about 13 times the value in comet 67P and higher than any methane measured in the Solar System.

Why it matters: The result points to ice that formed in a very cold, metal-poor cloud long before our Solar System existed, and it gives scientists a rare chemical sample of planet-forming material from another star.

What happens next: Astronomers will test the result against new chemical models of cold, metal-poor clouds, and they expect more interstellar visitors to be found as new telescopes scan the sky.

The number that does not fit

To understand why astronomers are excited, it helps to know what deuterium is. It is hydrogen with an extra neutron, a heavier twin of the most common atom in the universe. Most hydrogen has no neutron at all. A tiny share is deuterium, and that share acts like a thermometer: it records how cold the place was when a molecule formed.

The Webb result says that in the methane of 3I/ATLAS, about one hydrogen atom in 31 is deuterium. Put another way, roughly one methane molecule in eight carries a deuterium atom instead of an ordinary hydrogen atom. Methane is a simple molecule, one carbon atom joined to four hydrogen atoms. Its heavy form, written CH3D, swaps one of those four hydrogens for deuterium.

That share is enormous by Solar System standards. The hydrogen gas from which our Sun formed had a deuterium share of only about 0.0021 percent, according to the figures in the new paper. Against that starting mix, the comet’s methane is enriched roughly 1,500 times, and it sits about 200 times above the deuterium level of Earth’s ocean water.

The only fair comparison comes from comet 67P/Churyumov-Gerasimenko, which the European Space Agency’s Rosetta spacecraft studied up close. Rosetta flew through 67P’s coma and measured its methane with a mass spectrometer, an instrument that sorts molecules by weight. It found a methane deuterium ratio of 0.241 percent. The new value for 3I/ATLAS is 13 times higher, plus or minus two.

How Webb read a comet’s chemistry

Webb watched from afar with its Near-Infrared Spectrograph, an instrument called NIRSpec that splits infrared light into colours, each colour marking a different molecule. The observations were made on 22 and 23 December 2025, when the comet was about 2.4 times as far from the Sun as Earth.

By then the comet had passed its closest point to the Sun in late October 2025, and sunlight was warming its ice so gas streamed off into the coma. Ordinary methane shines brightly in infrared light, and the team first modelled those bright lines. When they subtracted that model, the faint lines of heavy methane, CH3D, appeared in what was left.

From the brightness of each gas, the team worked out how many molecules were leaving the comet every second. Because each methane molecule has four hydrogen atoms, the deuterium ratio is the heavy-methane rate divided by four times the ordinary-methane rate. That calculation gives 3.18 percent.

The measurement was not easy. The comet is faint, and two of the five background exposures failed because stars drifted into the field of view. The result rests on two days of observing a single object, and no second team has yet repeated the heavy-methane detection. Those limits matter, and the researchers state them in their paper.

Why heavy hydrogen is a thermometer

Deuterium enrichment happens in the cold. In a cloud of gas near 10 kelvin, about minus 263 degrees Celsius, gases freeze onto tiny dust grains. Because deuterium is heavier than ordinary hydrogen, it is favoured in the chemical reactions that build molecules on those icy grains. The colder and longer the freeze, the more deuterium ends up locked into the ice.

The team reads the new number as a sign that the comet’s ices formed in a cold, low-metallicity interstellar or protostellar environment under a high rate of cosmic-ray bombardment. Low metallicity means the cloud held only a small share of elements heavier than helium, the kind of elements that stars forge and scatter when they die. A cloud like that resembles the early Milky Way, before generations of stars had enriched the galaxy with heavier elements.

Cosmic rays play a part too. These fast charged particles cross the galaxy and set off a chain of reactions that builds a charged molecule called H3+. When that ion meets a hydrogen molecule carrying deuterium, the deuterium tends to move onto the ion, and the deep cold rarely lets it go back. In a metal-poor cloud there is less carbon monoxide to break that chain, so the deuterium-boosting step runs for longer. The result is gas rich in deuterium, which lands on icy dust grains where methane is built one atom at a time.

There is a second pattern that supports the cold-origin story. In 3I/ATLAS, methane holds more deuterium than water does. Earlier work measured the comet’s water deuterium ratio at 0.98 percent, so methane is enriched about 3.2 times more than water, a pattern also seen in Solar System bodies and in the cold gas between stars.

The comparison with comet 67P

Comet 67P is the benchmark because it is the only other comet with a secure heavy-methane detection. Rosetta orbited 67P for two years and sampled its coma directly, something no telescope can match. Webb’s remote reading of a far fainter object now sits far above it.

No published chemical model of a planet-forming disc predicts a methane deuterium ratio above 1 percent, and the models that exist assume metal levels like those of our own galactic neighbourhood. The authors of the new paper say it is uncertain whether any model can reproduce their number, and they note that models of deuteration in metal-poor clouds under strong cosmic rays have only recently started to be investigated.

Cold clouds between the stars come closer. Nobody has measured methane deuterium directly beyond the Solar System, so the team used a proxy molecule common in cold gas. Across 27 cold cores, estimates based on that proxy span 0.25 to 5.6 percent, and in one cloud they could reach 7 percent. Only four sources match or beat 3I/ATLAS.

The comet also carries a complication that the paper leaves unresolved. As 3I/ATLAS approached the Sun, studies of its coma found extra carbon dioxide, nickel, iron and methanol. The paper notes that those findings may point to metal-rich origins, the opposite of what the isotopes favour. The two clues do not yet agree, and that disagreement is part of the story. The isotope evidence in four different molecules points one way, while the metals point the other.

How old is it, really?

Some headlines have said the comet may contain material 10 billion years old, or that it formed more than 7 billion years before our Sun. Those numbers need careful handling, because the methane paper itself does not measure an age.

The age comes from a different study, led by Martin Cordiner and published in the journal Nature, which examined the comet’s isotopes and concluded that they require formation in a cold, metal-poor system as long as 10 to 12 billion years ago. The methane result fits that picture but cannot date anything on its own.

One of the clues behind the age is carbon. Carbon-13, the heavier stable form of carbon, builds up in the Milky Way as stars make it. A body born very early should therefore hold little of it, and measurements of 3I/ATLAS found low carbon-13 in its gases, which fits an early birth. Earlier studies based on the comet’s speed and arrival direction had already placed its birth in a metal-poor system more than 3 billion years ago.

None of this identifies the comet’s home star. The paper does not say which star 3I/ATLAS came from, and astronomers may never know. What the isotopes say is that the star formed in a part of the young galaxy that had not yet been enriched by many generations of stars.

A visitor that will not return

3I/ATLAS was discovered on 1 July 2025 by the ATLAS survey telescope at Rio Hurtado in Chile, a NASA-funded network that scans the sky for moving objects. It was only the third object ever confirmed to have come from interstellar space, after the rocky body 1I/’Oumuamua in 2017 and the comet 2I/Borisov in 2019. The “3I” in its name marks it as the third, and the “I” stands for interstellar.

Its path is open, not a closed loop around the Sun. It came from the direction of the constellation Sagittarius, swung past the Sun, and is now heading back into the dark. NASA notes that it will soon leave our Solar System and never return, but the observations will live on in the agency’s public data archives, gathered by more than a dozen NASA missions.

The comet also became famous for the wrong reasons. Within weeks of its discovery, claims spread on social media that it might be an alien spacecraft or probe, helped by a non-peer-reviewed paper from Harvard astrophysicist Avi Loeb. At a press briefing on 19 November, Nicky Fox, associate administrator of NASA’s Science Mission Directorate, said the agency had not seen any technosignatures, or signs of technology, from the comet. “We certainly haven’t seen any technosignatures or anything from it that would lead us to believe it was anything other than a comet,” she said. Other astronomers agreed that its gas, dust and motion look like those of an ordinary comet, even if its chemistry is extraordinary.

What scientists will look for next

The immediate work is on paper, not in the sky. Chemists will build new models of cold, metal-poor clouds under heavy cosmic-ray bombardment and see whether any of them can reach 3.18 percent. If none can, the models are missing something about how methane forms in the coldest corners of the galaxy.

The second line of work is to find more visitors. NASA estimates that an interstellar object may pass through our Solar System about once a year, and with more powerful telescopes coming into use, such discoveries are expected to become more common. Each new object is a fresh sample of another planetary system, and comparing them will show whether 3I/ATLAS is a rare freak or a typical product of the early galaxy.

For now, 3I/ATLAS is the most thoroughly studied interstellar object ever seen. Its methane has given scientists a number that no model predicted and no Solar System object matches. That is how progress often starts in astronomy: not with an answer, but with a measurement that refuses to fit.

Frequently asked questions

What is 3I/ATLAS?

3I/ATLAS is a comet from outside our Solar System, discovered on 1 July 2025 by the ATLAS survey telescope in Chile. It is the third interstellar object ever confirmed, after 1I/’Oumuamua in 2017 and 2I/Borisov in 2019. It has an icy nucleus and a coma of gas and dust, like comets from our own Solar System, but it formed around another star.

What did the James Webb Space Telescope find in its methane?

Webb found that the comet’s methane has a deuterium-to-hydrogen ratio of 3.18 percent, about 13 times the value in comet 67P and higher than any methane measurement in the Solar System. Roughly one methane molecule in eight carries a heavy hydrogen atom.

What is deuterium, and why does it matter?

Deuterium is a heavier form of hydrogen, with one extra neutron. Its abundance in a molecule records how cold the place was when the molecule formed, because cold clouds favour deuterium in the chemistry that builds molecules on icy dust grains.

Is 3I/ATLAS really 10 billion years old?

That age is an inference, not a direct measurement. An earlier study of the comet’s isotopes, led by Martin Cordiner and published in Nature, argued that it formed in a cold, metal-poor system as long as 10 to 12 billion years ago. The methane result fits that picture but does not measure an age by itself.

Is 3I/ATLAS an alien spacecraft?

There is no evidence for that. NASA has said it has seen no technosignatures, or signs of technology, from the object, and astronomers who studied its gas, dust and motion say it behaves like an ordinary comet.

Will 3I/ATLAS come back?

No. It follows an open path that will carry it out of our Solar System, never to return. Its closest approach to the Sun was in late October 2025, and it is now heading back into interstellar space.

Conclusion

A single measurement from a distant comet has given astronomers a glimpse of chemistry from the early galaxy. The methane of 3I/ATLAS holds more heavy hydrogen than any comparable measurement ever made, and it points to ice frozen in a cloud far colder and older than the one that built our Solar System. The result is not the final word: it rests on two days of observing one object, and some of the comet’s other clues point in a different direction. But that is what makes it valuable. A number that no model can explain is an invitation to build better models.

This article is for general information and education only. It is not legal or financial advice.

Sources

Faizan Altaf

Faizan Altaf is the founder and author of findivoza.com, a news blog covering world, Pakistan, business, technology, health and sport in plain English. He writes for general readers who want clear, trustworthy news without jargon. His work focuses on explaining what happened, why it matters, and what to watch next.

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